1. Field of the Invention
The present invention relates to a device and a method for non-contractual measurement of the speed of an object moving over a surface, and in particular to a device and a method for non-contractual speed measurement over surfaces by means of the Doppler principle making use of microwave or ultrasonic sensors, in particular for joggers or inline skaters.
2. Description of Prior Art
Within the last few years, performance diagnostic has become increasingly common in the field of leisure-time sports. Instruments which were previously the preserve of top sportsmen alone have recently been used more and more by the masses of leisure-time sportsmen. The heart-rate measuring unit, which, in view of the high price, was exclusively used by top sportsmen a few years ago, whereas it is now used by a large number of leisure-time sportsmen for personal health control, can be mentioned as an example in this connection.
In addition to health control there is, however, also an increasing need for assessing the respective performance achieved. In the case of cycling, this is done e.g. by indicating in the bicycle tachometer the speed and the distance covered. In the case of some other kinds of sport, e.g. jogging, inline skating and also downhill skiing or cross-country skiing, this information is not provided due to non-existing measurement methods or measurement methods which entail major disadvantages.
All the hitherto known methods for non-contractual speed measurement in the field of leisure time are either very susceptible to faults and inaccurate, when the methods in question are mechanical methods, or they entail restrictions with respect to the sensors used, e.g. microwave sensors or ultrasonic sensors.
A method of non-contractual speed measurement, in particular for skiers, is disclosed in U.S. Pat. No. 4,757,714. This method makes use of a speed sensor and of a data display fixed to a helmet. A transmitter is fixed to one ski and transmits ultrasonic or electromagnetic waves in the direction of the stationary medium over which a skier moves. An element of the same transmitter or a separate transducer detects waves reflected from the stationary medium. A computer calculates the speed of the skis from the Doppler shift of the reflected signal and activates a display unit so as to display the speed.
This kind of non-contractual speed measurement over surfaces by means of the Doppler principle is problematic insofar as the Doppler-shifted frequency depends on the angle between the ultrasonic sensor or the microwave sensor and the surface in motion relative to the sensor, the speed to be measured being the relative speed between the surface and the sensor.
In the prior art it is known to use two sensors, which are displaced by 90xc2x0 relative to one another, so as to compensate the angular dependency of the sensors. This kind of arrangement is known as Janus arrangement. The disadvantage of this arrangement is, on the one hand, that it cannot be employed in all cases of use, since the two sensors have to be fixed at an angle of 90xc2x0 to the direction of motion. This mode of an arrangement is not possible for joggers, by way of example. In addition, the use of two sensors is necessary, which will, on the one hand, entail higher costs and, on the other hand, fluctuations or drifts, respectively, of the sound will occur more easily, particularly for a transducer pointing backwards; such drifts adversely affect the accurate measurement of high speeds.
In a further known system for non-contractual speed measurement, which is described in DE 19501228, a sensor is used which exhibits a broad radiation characteristic so as to keep the influence of the angular dependency relative to the surface low. As disclosed in this publication, the influence of the angular dependency relative to the surface can, however, only be kept low if the irradiation angle is smaller than half the aperture angle of the sensor radiating the signal. The disadvantage of this known speed measurement are the necessary sensors which must exhibit a broad radiation characteristic. The frequencies of the sensors which have to be used as well as the maximum radiant power are determined by the Federal Bureau for Telecommunications. This means that if a sensor has a large aperture angle, this being the kind of sensor required according to DE 19501228, it will only be possible to obtain a limited power density and, consequently, maximum range of the sensor in view of the broad radiation characteristic. This means, in other words, that the maximum possible range can be achieved by a very narrow radiation characteristic.
For applications in the field of leisure-time sports, e.g. jogging or inline skating, the sensor is normally provided one meter above the ground so that a high radiant power density is required. The speed measurement system disclosed in DE 19501228 is, however, often not able to achieve such a radiant power density in view of the comparatively large radiation angle and the maximum admissible radiant power. It follows that the method described in DE 19501228 is particularly suitable for ski speed measurement when the sensor is provided in or on the ski, whereas in the case of other leisure-time sport activities, such as jogging or inline skating, where the sensor is arranged at a comparatively large distance from the ground, this measurement method will normally not provide any useful measurement results. A further disadvantage of the measurement system described in DE 19501228 is that sensors having a large radiation angle are normally much more expensive and much less easily available than narrow-cone sensors.
It is the object of the present invention to provide a device and a method for non-contractual measurement of the speed of an object moving over a surface, which provide, on the one hand, measurement results with high precision and, on the other hand, a high flexibility with regard to the field of use.
According to a first aspect of the present invention this object is achieved by a device for non-contractual measurement of the speed of an object moving over a surface which comprises a means for radiating a signal of fixed frequency at an angle onto the surface in or against the direction of motion, the angle being variable by the movement of the object, and for receiving a Doppler-shifted signal reflected at the surface. Furthermore, a means is provided for combining a plurality of reflected Doppler-shifted signals, which are received in temporal succession, so as to produce a combined spectrum. The device for non-contractual speed measurement additionally comprises a means for detecting from the combined spectrum the spectral portion having the highest or lowest frequency and exceeding a predetermined signal power, and a means for deducing the speed from the frequency of the detected spectral portion.
According to preferred embodiments of the present invention, a means is provided for down-converting to the baseband the received Doppler-shifted signal which has been reflected at the surface. When the received signal is down-converted to the baseband, the spectral portion having the highest frequency and exceeding a predetermined signal power is detected independently of the radiation direction of the sensor. Otherwise, e.g. when the received signal is converted to an intermediate frequency, the spectral portion having the highest frequency will be detected when the sensor radiates in the direction of motion, and the spectral portion having the lowest frequency will be detected when the sensor radiates against the direction of motion.
According to the present invention, a sensor having a narrow radiation characteristic can be used, which is mounted such that it either faces the direction of motion or faces away from the direction of motion. When the surface over which an object moves is irradiated, a spectral portion of the Doppler-shifted signal will occur due to the diffuse reflection of the sensor signal at the surface, this spectral portion being, however, dependent on the irradiation angle. This spectral portion is the highest frequency portion appearing in the Doppler spectrum, this frequency portion constituting then a direct measure for the speed. In order to compensate the dependence of the output signal on the angle between the sensor and the surface, averaging over a plurality of transmitting intervals is carried out according to the present invention.
A natural change of angle will occur in the case of a preferred field of use of the present invention, viz. speed detection in the field of leisure-time sports, e.g. jogging or inline skating, due to the movement of the leisure-time sportsman who carries the transmitter. By means of the averaging over time according to the present invention this natural change of angle, which results from the circumstance that the sensor is not rigidly mounted at a fixed angle relative to the surface, is utilized for detecting a signal component that extends almost parallel to the surface. This signal component mirrors the actual speed component. This averaging over a plurality of successive measurement periods and the utilization of the natural movement of the sensor, which results from the use of the sensor, leads to a temporal expansion of the radiation angle without any loss of range. Such a loss of range would occur if sensors with broader radiation angles were used.
For evaluating the combined spectrum, which is obtained from the individual spectra of the received, reflected, Doppler-shifted spectra, the spectral portion having the highest frequency or the xe2x80x9cfalling edgexe2x80x9d of the combined spectrum is ascertained. On the other hand, the spectral portion having the lowest frequency or the xe2x80x9crising edgexe2x80x9d of the Doppler spectrum is detected when the sensor radiates against the direction of motion and when the received signal is not converted to the baseband but e.g. to an intermediate frequency. The respective spectral portion can be detected by means of traditional spectral analysis methods or by a Fourier transform. Furthermore, the method described in DE 19501228 can be used for obtaining the respective spectral portion from the combined spectrum.
According to preferred embodiments of the present invention, the predetermined signal power which is used for detecting the spectral portion having the highest and the lowest frequency, respectively, is set on the basis of specific properties of the means for radiating and receiving, these specific properties including preferably the l/f noise of the means for radiating and receiving, i.e. of the sensor. Furthermore, preferred embodiments of the present invention include a means for correcting the deduced speed so as to compensate a speed-dependent curve shape of the combined spectrum. This speed-dependent correction is advantageous insofar as inaccuracies caused by the varying falling or rising edge of the spectrum are compensated for as the speed increases. This speed-dependent correction can specially be adapted to each type of sensor. Due to the continual motion imparted to the sensor by the person carrying the sensor, there will be fluctuations in the distance between the sensor and the surface on which this person is moving. Signal level differences resulting from this varying distance can be compensated for by an amplifier with automatic gain control.
According to a second aspect the object underlying the present invention is achieved by a method for non-contractual measurement of the speed of an object moving over a surface in the case of which a signal of fixed frequency is radiated at an angle onto the surface in or against the direction of motion, the angle being variable by the movement of the object. A Doppler-shifted signal reflected at the surface is received. The above-mentioned steps are repeated so as to produce a plurality of reflected Doppler-shifted signals received in temporal succession. The plurality of reflected Doppler-shifted signals received in temporal succession is combined so as to produce a combined spectrum. The spectral portion having the highest or lowest frequency and exceeding a predetermined signal power is detected from the combined spectrum, whereupon the speed is deduced from the frequency of the detected spectral portion.
It follows that the devices and methods according to the present invention permit non-contractual speed measurement over surfaces making use of standard sensors, microwave sensors or ultrasonic sensors having narrow transmit and receive characteristics, the measurement results provided by the present invention being extremely precise. The present invention additionally permits non-contractual speed measurement for leisure-time sports, e.g. jogging or inline skating, in the case of which such precise measurement has not been possible up to now.
Further developments of the present invention are disclosed in the dependent claims.